1. Field of the Invention
The present invention relates to a process for consolidation of a porous preform for an optical fiber by heating the preform. In particular, the present invention relates to an improved process for the consolidation of a porous glass preform in which the preform is heated in an atmosphere comprising a reduced pressure or a vacuum pressure so that a consolidated preform having less and impurities is stably produced over an extended period whereby an optical fiber having a good transmission properties can be produced from the consolidated preform.
2. Description of the Related Art
A glass preform comprising fine glass particles produced by the Vapor Deposition Method such as the Vapor Phase Axial Deposition (VAD) Method or the Outside Vapor Deposition (OVD) Method is heated to an elevated temperature in an electrical furnace so that it is consolidated to be a glass article. Conventionally, such a consolidation process is carried out by traversing the preform across a narrow heating zone at a normal pressure under an atmosphere comprising an inert gas such as He (helium) optionally containing a slight amount of a halogen (particularly chlorine) gas. The process may be referred to as the Zone Heating Process. Alternatively, the preform is consolidated by placing it in an electrical furnace having a large heating space enough to heat the whole preform uniformly and increasing a furnace temperature gradually. The process may be referred to as the Temperature Holding Process.
A problem in the conventional consolidation processes as described above, there is a problem during the heating consolidation treatment of the preform that voids (which will be, herein, referred to as "bubbles") are left in a produced glass article. Such bubbles are generated from gases originally confined between the fine glass particles and/or dissolved in the glass particles during the consolidation. In addition, the bubbles may be sometimes produced during a process at a high temperature after the consolidation. Japanese Patent Kokai Publication Nos. 201025/1988 and 275441/1989 propose processes for the consolidation of the preform under a reduced pressure or a vacuum pressure. In these processes, it can be expected that nearly no gas (thus, bubble) remains in the resulted glass article because the reduced or the vacuum atmosphere removes the gases in the porous preform for the optical fiber. The term "reduced pressure" herein used is intended to mean a pressure less than 5 kPa. The term "vacuum pressure" herein used is intended to mean a pressure less than 5 Pa without gas supply nor evacuation rate control.
One example of an apparatus is shown in FIG. 2 which can be used in the conventional process for the consolidation of the preform for the optical fiber under the reduced pressure or the vacuum pressure. In FIG. 2, the numerical number 31 indicates a furnace body (or a pressure vessel), 32 does a muffle tube, 33 does a heater, 34 does a heat shield 35 does a gas inlet, 36 does a gas outlet and 37 does a vacuum pump.
When a porous preform 38 for an optical fiber is consolidated, it is held in the muffle tube 32 and heated to a temperature not higher than 1700.degree. C. while the furnace body 31 is maintained at a vacuum pressure by evacuating the furnace body using the vacuum pump 37. Optionally, any required gas can be added in and withdrawn from the pressure vessel 31 through the inlet 35 and the outlet 36 with using the vacuum pump 37.
Some relationships between a gas supply to the furnace, a gas evacuation from the furnace and a heating temperature increasing rate pattern are disclosed for the consolidation using the above apparatus.
Examples of the relationships described in Japanese Patent Kokai Publication No. 201025/1988 are shown as follows:
__________________________________________________________________________ (1) Consolidation process under vacuum pressure: Temperature: From R.T.* to 1000.degree. C. From 1000.degree. C. to 1600.degree. C. *(Room Temperature) Temp. Increasing Rate: 20.degree. C./min. 5.degree. C./min. Pressure: &lt;0.65 Pa &lt;1.3 Pa (without evacuation rate control) Gas Supply: Non Non (2) Consolidation process under reduced pressure: Temperature: From R.T. to 1000.degree. C. From 1000.degree. C. to 1600.degree. C. Temp. Increasing Rate: 20.degree. C./min. 5.degree. C./min. Pressure: 13 Pa 13 Pa (with evacuation rate control) Gas Supply: Ar 10 scc*/min. Ar 10 scc/min. *(standard cubic centimeter) __________________________________________________________________________
Other examples of the relationship as described in Japanese Patent Kokai Publication No. 275441/1989 are shown as follows:
__________________________________________________________________________ (3): Temperature: From R.T. to 1600.degree. Temperature Increasing Rate: 8.degree. C./min. Pressure: 10 Pa (with evacuation rate control) Gas Supply: He 200 scc/min. (4): Temperature: From R.T. to 1000.degree. C. From 1000.degree. C. to 1600.degree. C. Temp. Increasing Rate: 6.degree. C./min. 8.degree. C./min. Pressure: 10 Pa 10 Pa (with evacuation rate control) Gas Supply: Cl.sub.2 100 scc/min. He 100 scc/min. __________________________________________________________________________
In each of the processes in which the above relationships (2), (3) and (4) are carried out, the gas was supplied to the inside of the muffle tube.
In the process in which the conventional apparatus is used for the consolidation, a certain amount of a gas is supplied to the furnace while a evacuation rate of the gas from the furnace is controlled in order that the pressure in the furnace is maintained constant.
However, the preform before being thermally treated includes and/or adsorbs a large amount of water and oxygen therein, which are liberated into the inside of the furnace when the preform is thermally treated, whereby carbon material which is often used for the muffle tube, the heater and also the heat shield of the heater is exhausted due to oxidation caused by water and oxygen. In addition, members constituting the furnace which are made of a stainless steel material are also oxidized and degraded.
Such oxidation and/or degradation may not be a problem if the furnace is only used for a short period of time, but there arise the following problems when the furnace is used for the vitrification of a number of the preforms over a long period of time:
a. The exhausted carbon members must be replaced; PA1 b. Powder which is generated on the exhaustion of the carbon material may attach to the glass preform, which causes bubble generation in the preform; PA1 c. When the furnace is degraded due to the oxidation, the rust may fall down in the muffle tube and be mixed into the preform, which inversely affects the transmission properties of the optical fiber which is produced from the preform; and PA1 d. In connection with the above "c", removal of the rust must be often carried out very carefully.
In addition, when the glass preform for the optical fiber is dehydrated using a halogen gas or a halide gas, a product gas is generated by a reaction of the halogen gas or the halide gas with water liberated from the preform. For example, when the halogen gas is chlorine gas, hypochlorous acid is generated. Oxidation of the carbon material or the stainless steel material is enhanced by such a product gas compared with the case in which only water is present. This problem is very critical when the halogen gas or the halide gas is used.